WASHINGTON — President Barack Obama waged a fierce fight to pass his health care law four years ago. But as his administration prepares to put it in place, he is facing an aggressive Republican campaign to prevent a successful rollout and to deny him his most important legacy.

Starting this week, the White House will kick off a six-month campaign to persuade millions of uninsured Americans to sign up for health coverage as part of insurance marketplaces that open for business Oct. 1. If too few people enroll, the centerpiece of the president’s Affordable Care Act could collapse.

But instead of offering the kind of grudging cooperation that normally follows even the most bitter of legislative battles, Obama’s foes have intensified their opposition, trying to deepen the nation’s anger about the health insurance program, which both sides often call Obamacare.

Across the country, Republicans are eager to prevent people from enrolling, fearing that once people begin receiving the benefit they will be loath to give it up. And in Washington, lawmakers have cast the law as the evil villain in a legislative melodrama about the budget that is barreling toward another government shutdown.

One group called Generation Opportunity distributed a Web video last week showing a creepy-looking Uncle Sam peering between a woman’s legs at a gynecologist’s office.

“Don’t let government play doctor,” the video says at the end. “Opt out of Obamacare.”

In the face of the intense opposition, the White House is pushing ahead with a fierce public relations effort that will begin ramping up in earnest Monday, according to top White House aides in charge of the program.

Officials said the rollout would include a presidential event next week in New York with former President Bill Clinton and a health care speech by Obama on Thursday in Maryland. Michelle Obama will urge mothers and veterans to get their families enrolled. Vice President Joe Biden will host a nationwide conference call with nurses to enlist them in the effort to spread the word. Members of the president’s cabinet will fan out across the country, lobbying constituent groups to prod their members into action.

Those efforts will eventually be augmented by a Madison Avenue-style advertising campaign by insurance companies, which officials say are poised to spend $1 billion or more to attract millions of new customers. Some of the ads are likely to be aimed at young people, many of whom are uninsured but healthy — and great for the insurance companies’ bottom line.

Liberal advocacy groups have also begun to organize door-to-door canvassing much as they did on behalf of Obama’s two presidential campaigns.

The overarching goal is to persuade many of today’s 48 million uninsured to sign up for insurance on the new exchanges created by the law.

Crucially, officials need to woo older, sicker people without insurance as well as younger, healthier people, whose payments effectively subsidize those who will end up using more health care. Obama’s organizing strength among young adults — whom he won by a wide margin in 2008 and 2012 — may aid the campaign.

“We will have to create buzz and engagement and adjust and reach people in a sustained way from October to the end of March,” said Tara McGuinness, who is leading the health care communications effort inside the White House. “This is about what makes sense for families, what’s affordable for them.”

But even as Obama’s campaign accelerates, Republicans at all political levels are working against the law.

The Republican National Committee has begun what it calls a monthlong “awareness campaign,” with a television booking operation to make sure that pundits opposed to the law are always available to counter its boosters. The committee’s effort has already booked local and national politicians on radio programs like “The Hugh Hewitt Show” and cable TV programs like “The Mike Huckabee Show.”

A Republican committee website counts down the days, hours, minutes and seconds until what it calls the “Obamacare Train Wreck.”

Other conservative groups are broadcasting TV advertisements that urge people not to sign up for the health care law. Americans for Prosperity began broadcasting an ad last week featuring a cancer survivor who warns about the dangers of the law. It is the latest in a series of TV commercials featuring women criticizing the law.

“Obamacare is dangerous. It can’t be implemented,” Tricia, the cancer survivor, says in the ad. “Your well-being judged by a bureaucrat in D.C. is devastating.”

Republican state and local officials are trying to thwart the administration’s enrollment efforts by imposing restrictions and requirements on volunteers. The Heritage Action Fund organized a “Defund Obamacare” bus tour this summer that helped convince House Republicans that no budget deal should be made without stripping the money from the health law.

And in Congress, House Republicans are threatening to shut down the government and risk a default unless Congress eliminates all of the financing for the law, effectively killing it.

“Today, the constitutional conservatives in the House are keeping their word to our constituents and our nation to stand true to our principles, to protect them from the most unpopular law ever passed in the history of the country — Obamacare — that intrudes on their privacy and our most sacred right as Americans to be left alone,” Rep. John Culberson, R-Texas, said on the House floor Friday.

White House officials bluntly call the Republican efforts a “sabotage campaign” and concede that the assault on the law will make it harder to persuade people to sign up for insurance. In Florida, Ohio and Missouri, state officials have already moved to undercut efforts to enroll people in coverage. In Georgia, the state insurance commissioner, Ralph T. Hudgens, has said he will do “everything in our power to be an obstructionist.”

The result, White House officials said, is likely to be confusion among some members of the public and the potential for a muddled message at exactly the worst time. People may get a knock on the door from someone urging them to sign up for insurance, only to see a TV commercial a few minutes later urging them to “opt out” of the program.

“It’s rather extraordinary, if you think about it, that there are efforts underway to prevent Americans from getting benefits that they lawfully could enjoy and should enjoy,” said Jay Carney, the White House press secretary.

White House aides remain optimistic that some of what they describe as Republican antics will not be seen by the typical American. They are betting that support from Hollywood celebrities like Jennifer Hudson, Kal Penn and Amy Poehler — all of whom met with Obama this summer to lend their support for the health care campaign — will be more influential than Republican politicians. Singer Katy Perry, who has 45 million Twitter followers (more than Obama’s 37 million) urged followers to check out the options at last month’s Video Music Awards ceremonies.

They are also counting on a network of volunteers, partly directed by Organizing for America, the president’s former campaign operation, to knock on thousands of doors in the weeks and months ahead.

Last week, Obama joined a conference call with thousands of health law volunteers to encourage them in their efforts.

“So we’re on our way to make sure that health care is affordable for every single American, and they’re not using the emergency room as their primary care provider,” Obama told the volunteers. “But that only happens with all of you.”